High Starch Light Weight Gypsum Wallboard

ABSTRACT

The invention generally provides gypsum-containing slurries including stucco, naphthalenesulfonate dispersant, and pregelatinized starch. The naphthalenesulfonate dispersant is present in an amount of about 0.1%-3.0% by weight based on the weight of dry stucco. The pregelatinized starch is present in an amount of at least about 0.5% by weight up to about 10% by weight of pregelatinized starch by weight based on the weight of dry stucco in the formulation. Other slurry additives can include trimetaphosphate salts, accelerators, binders, paper fiber, glass fiber, and other known ingredients. The invention also comprises the gypsum-containing products made with such slurries, for example, gypsum wallboard, and a method of making gypsum wallboard.

This application is a continuation of U.S. patent application Ser. No.11/449,177, filed Jun. 7, 2006, which claims priority to U.S.Provisional Patent Application No. 60/688,839, filed Jun. 9, 2005, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to a method making gypsum slurries containing apregelatinized starch and a naphthalenesulfonate dispersant, andproducts made therefrom. It also pertains to a method of increasing drystrength of gypsum-containing products including wallboard by using anaphthalenesulfonate dispersant in combination with pregelatinizedstarch in the slurry used to make the products.

BACKGROUND OF THE INVENTION

Certain properties of gypsum (calcium sulfate dihydrate) make it verypopular for use in making industrial and building products, such asgypsum wallboard. Gypsum is a plentiful and generally inexpensive rawmaterial which, through a process of dehydration and rehydration, can becast, molded or otherwise formed into useful shapes. The base materialfrom which gypsum wallboard and other gypsum products are manufacturedis the hemihydrate form of calcium sulfate (CaSO₄.1/2H₂O), commonlytermed “stucco,” which is produced by heat conversion of the dihydrateform of calcium sulfate (CaSO₄.2H₂O), from which 1½ water molecules beenremoved.

Conventional gypsum-containing products such as gypsum wallboard havemany advantages, such as low cost and easy workability, althoughsubstantial amounts of gypsum dust can be generated when the productsare cut or drilled. Various improvements have been achieved in makinggypsum-containing products using starches as ingredients in the slurriesused to make such products. Starch can increase flexural strength andcompressive strength of gypsum-containing products including gypsumwallboard. Known gypsum wallboard contains board starch at levels ofless than about 10 lbs/MSF.

It is also necessary to use substantial amounts of water in gypsumslurries containing pregelatinized starch in order to ensure properflowability of the slurry. Unfortunately, most of this water musteventually be driven off by heating, which is expensive due to the highcost of the fuels used in the heating process. The heating step is alsotime-consuming. It has been found that the use of naphthalenesulfonatedispersants can increase the fluidity of the slurries, thus overcomingthe water demand problem. In addition, it has also been found that thenaphthalenesulfonate dispersants, if the usage level is high enough, cancross-link to the pregelatinized starch to bind the gypsum crystalsafter drying, thus increasing dry strength of the gypsum composite.Trimetaphosphate salts have not in the past been recognized to affectgypsum slurry water requirements. However, the present inventors havediscovered that increasing the level of the trimetaphosphate salt tohitherto unknown levels in the presence of a specific dispersant makesit possible to achieve proper slurry flowability with unexpectedlyreduced amounts of water, even in the presence of high starch levels.This, of course, is highly desirable because it in turn reduces fuelusage as well as the process time associated with subsequent waterremoval process steps. Thus the present inventors have also discoveredthat the dry strength of gypsum board can be increased by using anaphthalenesulfonate dispersant in combination with pregelatinizedstarch in the slurry used to make the wallboard.

BRIEF SUMMARY OF THE INVENTION

The invention generally comprises a slurry including stucco,naphthalenesulfonate dispersant, and pregelatinized starch. Thenaphthalenesulfonate dispersant is present in an amount of about0.1%-3.0% by weight based on the weight of dry stucco. Thepregelatinized starch is present in an amount of at least about 0.5% byweight up to about 10% by weight based on the weight of dry stucco inthe formulation. Other slurry additives can include accelerators,binders, paper or glass fibers and other known constituents. Theinvention also comprises the gypsum-containing products made with suchslurries.

The invention also comprises a slurry including stucco, trimetaphosphatesalt, naphthalenesulfonate dispersant, and pregelatinized starch. Thesodium trimetaphosphate is present in an amount of at least about 0.12%by weight based on the weight of stucco. In a preferred embodiment, thetrimetaphosphate salt is present in an amount of about 0.12-0.4% byweight based on the weight of dry stucco. The naphthalenesulfonatedispersant is present in an amount of about 0.1%-3.0% by weight based onthe weight of dry stucco. The pregelatinized starch is present in anamount of at least about 0.5% by weight up to about 10% by weight basedon the weight of dry stucco in the formulation. Other slurry additivescan include accelerators, binders, paper or glass fibers and other knownconstituents. The invention also comprises the gypsum-containingproducts made with such slurries.

A preferred gypsum-containing product is gypsum wallboard. In thisembodiment, the invention constitutes gypsum wallboard comprising a setgypsum composition formed between two substantially parallel coversheets, the set gypsum composition made using the gypsum-containingslurry of water, stucco, pregelatinized starch, and anaphthalenesulfonate dispersant. The gypsum-containing slurry canoptionally contain a trimetaphosphate salt, for example, sodiumtrimetaphosphate. This gypsum wallboard made in accordance with theinvention has a high strength, yet much lower weight than conventionalboards. In addition, much less dust is generated on cutting, sawing,snapping, or drilling the wallboards made according to this embodiment.

In another embodiment the invention constitutes a method of makinggypsum wallboard by mixing a gypsum-containing slurry comprising water,stucco, pregelatinized starch, and a naphthalenesulfonate dispersant,wherein the pregelatinized starch is present in an amount of at leastabout 0.5% by weight up to about 10% by weight based on the weight ofstucco. The resulting gypsum-containing slurry is deposited on a firstpaper cover sheet, and a second paper cover sheet is placed over thedeposited slurry to form a gypsum wallboard. The gypsum wallboard is cutafter the gypsum-containing slurry has hardened sufficiently forcutting, and the resulting gypsum wallboard is dried. Thegypsum-containing slurry can optionally contain a trimetaphosphate salt,for example, sodium trimetaphosphate. Other conventional ingredientswill also be used in the slurry including, as appropriate, accelerators,binders, paper fiber, glass fiber, and other known ingredients. A soapfoam is normally added to reduce the density of the final gypsumwallboard product.

DETAILED DESCRIPTION OF THE INVENTION

According to one embodiment of the present invention, there are providedfinished gypsum-containing products made from gypsum-containing slurriescontaining stucco, pregelatinized starch, and a naphthalenesulfonatedispersant. The naphthalenesulfonate dispersant is present in an amountof about 0.1%-3.0% by weight based on the weight of dry stucco. Thepregelatinized starch is present in an amount of at least about 0.5% byweight up to about 10% by weight based on the weight of dry stucco inthe formulation. Other ingredients that may be used in the slurryinclude binders, paper fiber, glass fiber, and accelerators. A soap foamis normally added to the newly formulated gypsum-containing slurries toreduce the density of the final gypsum-containing product, for example,gypsum wallboard.

Optionally, the combination of from about 0.5% by weight up to about 10%by weight pregelatinized starch, from about 0.1% by weight up to about3.0% by weight naphthalenesulfonate dispersant, and a minimum of atleast about 0.12% by weight up to about 0.4% by weight oftrimetaphosphate salt (all based on the weight of dry stucco used in thegypsum slurry) unexpectedly and significantly increases the fluidity ofthe gypsum slurry. This substantially reduces the amount of waterrequired to produce a gypsum slurry with sufficient flowability to beused in making gypsum-containing products such as gypsum wallboard. Thelevel of trimetaphosphate salt, which is at least about twice that ofstandard formulations (as sodium trimetaphosphate), is believed to boostthe dispersant activity of the naphthalenesulfonate dispersant.

The naphthalenesulfonate dispersants used in the present inventioninclude polynaphthalenesulfonic acid and its salts(polynaphthalenesulfonates) and derivatives, which are condensationproducts of naphthalenesulfonic acids and formaldehyde. Particularlydesirable polynaphthalenesulfonates include sodium and calciumnaphthalenesulfonate. The average molecular weight of thenaphthalenesulfonates can range from about 3,000 to 27,000, although itis preferred that the molecular weight be about 8,000 to 10,000. At agiven solid % aqueous solution, a higher molecular weight dispersant hashigher viscosity, and generates a higher water demand in theformulation, than a lower molecular weight dispersant. Usefulnaphthalenesulfonates include DILOFLO, available from GEO SpecialtyChemicals, Cleveland, Ohio; DAXAD, available from Hampshire ChemicalCorp., Lexington, Mass.; and LOMAR D, available from GEO SpecialtyChemicals, Lafayette, Ind. The naphthalenesulfonates are preferably usedas aqueous solutions in the range 35-55% by weight solids content, forexample. It is most preferred to use the naphthalenesulfonates in theform of an aqueous solution, for example, in the range of about 40-45%by weight solids content. Alternatively, where appropriate, thenaphthalenesulfonates can be used in dry solid or powder form, such asLOMAR D, for example.

The polynaphthalenesulfonates useful in the present invention have thegeneral structure (I):

wherein n is >2, and wherein M is sodium, potassium, calcium, and thelike.

The naphthalenesulfonate dispersant, preferably as an about 45% byweight solution in water, may be used in a range of from about 0.5% toabout 3.0% by weight based on the weight of dry stucco used in thegypsum composite formulation. A more preferred range ofnaphthalenesulfonate dispersant is from about 0.5% to about 2.0% byweight based on the weight of dry stucco, and a most preferred rangefrom about 0.7% to about 2.0% by weight based on the weight of drystucco. In contrast, known gypsum wallboard contains this dispersant atlevels of about 0.4% by weight, or less, based on the weight of drystucco.

Stated in an another way, the naphthalenesulfonate dispersant, on a dryweight basis, may be used in a range from about 0.1% to about 1.5% byweight based of the weight of dry stucco used in the gypsum compositeformulation. A more preferred range of naphthalenesulfonate dispersant,on a dry solids basis, is from about 0.25% to about 0.7% by weight basedon the weight of dry stucco, and a most preferred range (on a dry solidsbasis) from about 0.3% to about 0.7% by weight based on the weight ofdry stucco.

The gypsum-containing slurry can optionally contain a trimetaphosphatesalt, for example, sodium trimetaphosphate. Any suitable water-solublemetaphosphate or polyphosphate can be used in accordance with thepresent invention. It is preferred that a trimetaphosphate salt be used,including double salts, that is trimetaphosphate salts having twocations. Particularly useful trimetaphosphate salts include sodiumtrimetaphosphate, potassium trimetaphosphate, calcium trimetaphosphate,sodium calcium trimetaphosphate, lithium trimetaphosphate, ammoniumtrimetaphosphate, and the like, or combinations thereof. A preferredtrimetaphosphate salt is sodium trimetaphosphate. It is preferred to usethe trimetaphosphate salt as an aqueous solution, for example, in therange of about 10-15% by weight solids content. Other cyclic or acyclicpolyphosphates can also be used, as described in U.S. Pat. No. 6,409,825to Yu et al., herein incorporated by reference.

Sodium trimetaphosphate is a known additive in gypsum-containingcompositions, although it is generally used in a range of from about0.05% to about 0.08% by weight based on the weight of dry stucco used inthe gypsum slurry. In the embodiments of the present invention, sodiumtrimetaphosphate (or other water-soluble metaphosphate or polyphosphate)can be present in the range of from about 0.12% to about 0.4% by weightbased on the weight of dry stucco used in the gypsum compositeformulation. A preferred range of sodium trimetaphosphate (or otherwater-soluble metaphosphate or polyphosphate) is from about 0.12% toabout 0.3% by weight based on the weight of dry stucco used in thegypsum composite formulation.

There are two forms of stucco, alpha and beta. These two types of stuccoare produced by different means of calcination. In the presentinventions either the beta or the alpha form of stucco may be used.

Starches, including pregelatinized starch in particular, must be used ingypsum-containing slurries prepared in accordance with the presentinvention. A preferred pregelatinized starch is pregelatinized cornstarch, for example pregelatinized corn flour available from BungeMilling, St. Louis, Mo., having the following typical analysis: moisture7.5%, protein 8.0%, oil 0.5%, crude fiber 0.5%, ash 0.3%; having a greenstrength of 0.48 psi; and having a loose bulk density of 35.0 lb/ft³.Pregelatinized corn starch should be used in an amount of at least about0.5% by weight up to about 10% by weight, based on the weight of drystucco used in the gypsum-containing slurry.

The present inventors have further discovered that an unexpectedincrease in dry strength (particularly in wallboard) can be obtained byusing at least about 0.5% by weight up to about 10% by weightpregelatinized starch (preferably pregelatinized corn starch) in thepresence of about 0.1% by weight to 3.0% by weight naphthalenesulfonatedispersant (starch and naphthalenesulfonate levels based on the weightof dry stucco present in the formulation). This unexpected result can beobtained whether or not water-soluble metaphosphate or polyphosphate ispresent.

In addition, it has unexpectedly been found that pregelatinized starchcan be used at levels of at least about 10 lb/MSF, or more, in the driedgypsum wallboard made in accordance with the present invention, yet highstrength and low weight can be achieved. Levels as high as 35-45 lb/MSFpregelatinized starch in the gypsum wallboard have been shown to beeffective. As an example, Formulation B, as shown in Tables 1 and 2below, includes 45 lb/MSF, yet produced a board weight of 1042 lb/MSFhaving excellent strength. In this example (Formulation B), anaphthalenesulfonate dispersant as a 45% by weight solution in water,was used at a level of 1.28% by weight.

Other useful starches include acid-modified starches, such asacid-modified corn flour, available as HI-BOND from Bunge Milling, St.Louis, Mo. This starch has the following typical analysis: moisture10.0%, oil 1.4%, solubles 17.0%, alkaline fluidity 98.0%, loose bulkdensity 30 lb/ft³, and a 20% slurry producing a pH of 4.3. Anotheruseful starch is non-pregelatinized wheat starch, such as ECOSOL-45,available from ADM/Ogilvie, Montreal, Quebec, Canada.

A further unexpected result may be achieved with the present inventionwhen the naphthalenesulfonate dispersant trimetaphosphate saltcombination is combined with pregelatinized corn starch, and optionally,paper fiber or glass fiber. Gypsum wallboard made from formulationscontaining these three ingredients have increased strength and reducedweight, and are more economically desirable due to the reduced waterrequirements in their manufacture.

Accelerators can be used in the gypsum-containing compositions of thepresent invention, as described in U.S. Pat. No. 6,409,825 to Yu et al.,herein incorporated by reference. One desirable heat resistantaccelerator (HRA) can be made from the dry grinding of landplaster(calcium sulfate dihydrate). Small amounts of additives (normally about5% by weight) such as sugar, dextrose, boric acid, and starch can beused to make this HRA. Sugar, or dextrose, is currently preferred.Another useful accelerator is “climate stabilized accelerator” or“climate stable accelerator,” (CSA) as described in U.S. Pat. No.3,573,947, herein incorporated by reference.

The following examples further illustrate the invention. They should notbe construed as in any way limiting the scope of the invention.

Example 1 Sample Gypsum Slurry Formulations

Gypsum slurry formulations are shown in Table 1 below. All values inTable 1 are expressed as weight percent based on the weight of drystucco. Values in parentheses are dry weight in pounds (lb/MSF).

TABLE 1 Component Formulation A Formulation B Stucco (lb/MSF) (732)(704) sodium 0.20 (1.50) 0.30 (2.14) trimetaphosphate Dispersant 0.18(1.35) 0.58¹ (4.05) (naphthalenesulfonate) Pregelatinized starch 2.7(20) 6.4 (45) (dry powder) Board starch 0.41 (3.0)  0 Heat resistant (15)  (15) accelerator (HRA) Glass fiber 0.27 (2.0) 0.28 (2.0) Paperfiber  0 0.99 (7.0) Soap* 0.03 (0.192) 0.03 (0.192) Total Water (lb.)805 852 Water/Stucco ratio    1.10    1.21 *Used to pregenerate foam.¹1.28% by weight as a 45% aqueous solution.

Example 2 Preparation of Wallboards

Sample gypsum wallboards were prepared in accordance with U.S. Pat. Nos.6,342,284 to Yu et al. and 6,632,550 to Yu et al., herein incorporatedby reference. This includes the separate generation of foam andintroduction of the foam into the slurry of the other ingredients asdescribed in Example 5 of these patents.

Test results for gypsum wallboards made using the Formulations A and Bof Example 1, and a control are shown in Table 2 below. As in thisexample and other examples below, nail pull resistance, core hardness,and flexural strength tests were performed according to ASTM C-473.Additionally, it is noted that typical gypsum wallboard is approximately½ inch thick and has a weight of between about 1600 to 1800 pounds per1,000 square feet of material, or lb/MSF. (“MSF” is a standardabbreviation in the art for a thousand square feet; it is an areameasurement for boxes, corrugated media and wallboard.)

TABLE 2 Control Formulation A Formulation B Lab test result Board BoardBoard Board weight (lb/MSF) 1587 1066 1042 Nail pull resistance (lb)81.7 50.2 72.8 Core hardness (lb) 16.3 5.2 11.6 Humidified bond load(lb) 17.3 20.3 15.1 Humidified bond 0.6 5 11.1 failure (%) Flexuralstrength, 47 47.2 52.6 face-up (MD) (lb) Flexural strength, 51.5 66.778.8 face-down (MD) (lb) Flexural strength, 150 135.9 173.1 face-up(XMD) (lb) Flexural strength, 144.4 125.5 165.4 face-down (XMD) (lb) MD:machine direction XMD: across machine direction

As illustrated in Table 2, gypsum wallboards prepared using theFormulation A and B slurries have significant reductions in weightcompared to the control board. With reference again to Table 1, thecomparisons of the Formulation A board to the Formulation B board aremost striking. The water/stucco (w/s) ratios are similar in FormulationA and Formulation B. A significantly higher level ofnaphthalenesulfonate dispersant is also used in Formulation B. Also, inFormulation B substantially more pregelatinized starch was used, about6% by weight, a greater than 100% increase over Formulation Aaccompanied by marked strength increases. Even so, the water demand toproduce the required flowability remained low in the Formulation Bslurry, the difference being about 10% in comparison to Formulation A.The low water demand in both Formulations is attributed to thesynergistic effect of the combination of naphthalenesulfonate dispersantand sodium trimetaphosphate in the gypsum slurry, which increases thefluidity of the gypsum slurry, even in the presence of a substantiallyhigher level of pregelatinized starch.

As illustrated in Table 2, the wallboard prepared using the FormulationB slurry has substantially increased strength compared with thewallboard prepared using the Formulation A slurry. By incorporatingincreased amounts of pregelatinized starch in combination with increasedamounts of naphthalenesulfonate dispersant and sodium trimetaphosphate,nail pull resistance in the Formulation B board improved by 45% over theFormulation A board. Substantial increases in flexural strength werealso observed in the Formulation B board as compared to the FormulationA board.

Example 3 ½ Inch Gypsum Wallboard Weight Reduction Trials

Further gypsum wallboard examples (Boards C, D and E), including slurryformulations and test results are shown in Table 3 below. The slurryformulations of Table 3 include the major components of the slurries.Values in parentheses are expressed as weight percent based on theweight of dry stucco.

TABLE 3 Control Formulation Formulation Formulation Board C Board DBoard E Board Trial formulation component/parameter Dry stucco (lb/MSF)1300 1281 1196 1070 Accelerator (lb/MSF) 9.2 9.2 9.2 9.2 DILOFLO¹(lb/MSF) 4.1 (0.32%) 8.1 (0.63%) 8.1 (0.68%) 8.1 (0.76%) Regular starch(lb/MSF) 5.6 (0.43%) 0 0 0 Pregelatinized corn 0  10 (0.78%)  10 (0.84%) 10 (0.93%) starch (lb/MSF) Sodium trimetaphosphate 0.7 (0.05%) 1.6(0.12%) 1.6 (0.13%) 1.6 (0.15%) (lb/MSF) Total water/stucco 0.82 0.820.82 0.84 ratio (w/s) Trial formulation test results Dry board weight1611 1570 1451 1320 (lb/MSF) Nail pull resistance (lb) 77.3^(†) 85.577.2 65.2 ^(†)ASTM standard: 77 lb ¹DILOFLO is a 45% Naphthalensulfonatesolution in water

As illustrated in Table 3, Boards C, D, and E were made from a slurryhaving substantially increased amounts of starch, DILOFLO dispersant,and sodium trimetaphosphate in comparison with the control board (abouta two-fold increase on a percentage basis for the starch and dispersant,and a two- to three-fold increase for the trimetaphosphate), whilemaintaining the w/s ratio constant. Nevertheless, strength as measuredby nail pull resistance was not dramatically affected and board weightwas significantly reduced. Therefore, in this example of an embodimentof the invention, the new formulation (such as, for example, Board D)can provide increased starch formulated in a usable, flowable slurry,while maintaining adequate strength.

Example 4 Wet Gypsum Cube Strength Test

The wet cube strength tests were carried out by using Southard CKS boardstucco, available from United States Gypsum Corp., Chicago, Ill. and tapwater in the laboratory to determine their wet compressive strength. Thefollowing lab test procedure was used.

Stucco (1000 g), CSA (2 g), and tap water (1200 cc) at about 70° F. wereused for each wet gypsum cube cast. Pregelatinized corn starch (20 g,2.0% based on stucco wt.) and CSA (2 g, 0.2% based on stucco wt.) werethoroughly dry mixed first in a plastic bag with the stucco prior tomixing with a tap water solution containing both naphthalenesulfonatedispersant and sodium trimetaphosphate. The dispersant used was DILOFLOdispersant (1.0-2.0%, as indicated in Table 4). Varying amounts ofsodium trimetaphosphate were used also as indicated in Table 4.

The dry ingredients and aqueous solution were initially combined in alaboratory Warning blender, the mixture produced allowed to soak for 10sec, and then the mixture was mixed at low speed for 10 sec in order tomake the slurry. The slurries thus formed were cast into three 2″X2″X2″cube molds. The cast cubes were then removed from the molds, weighed,and sealed inside plastic bags to prevent moisture loss before thecompressive strength test was performed. The compressive strength of thewet cubes was measured using an ATS machine and recorded as an averagein pounds per square inch (psi). The results obtained were as follows:

TABLE 4 Sodium trimetaphos- Wet cube Wet cube Test phate, grams DILOFLO¹ weight compressive Sample (wt % based on (wt % based on (2″ × 2″ ×2″), strength, No. dry stucco) dry stucco) g psi 1 0 1.5 183.57 321 20.5 (0.05)  1.5 183.11 357 3 1 (0.1) 1.5 183.19 360 4 2 (0.2) 1.5 183.51361 5 4 (0.4) 1.5 183.65 381 6 10 (1.0)  1.5 183.47 369 7 0 1.0 184.02345 8 0.5 (0.05)  1.0 183.66 349 9 1 (0.1) 1.0 183.93 356 10 2 (0.2) 1.0182.67 366 11 4 (0.4) 1.0 183.53 365 12 10 (1.0)  1.0 183.48 341 13 02.0 183.33 345 14 0.5 (0.05)  2.0 184.06 356 15 1 (0.1) 2.0 184.3  36316 2 (0.2) 2.0 184.02 363 17 4 (0.4) 2.0 183.5  368 18 10 (1.0)  2.0182.68 339 ¹ DILOFLO is a 45% Naphthalensulfonate solution in water

As illustrated in Table 4, Samples 4-5, 10-11, and 17, having levels ofsodium trimetaphosphate in the about 0.12-0.4% range of the presentinvention generally provided superior wet cube compressive strength ascompared to samples with sodium trimetaphosphate outside this range.

Example 5 ½ Inch Light Weight Gypsum Wallboard Plant Production Trials

Further trials were performed (Trial Boards 1 and 2), including slurryformulations and test results are shown in Table 5 below. The slurryformulations of Table 5 include the major components of the slurries.Values in parentheses are expressed as weight percent based on theweight of dry stucco.

TABLE 5 Plant Plant Control Formulation Control Formulation Board 1Trial Board 1 Board 2 Trial Board 2 Trial formulationcomponent/parameter Dry stucco (lb/MSF) 1308 1160 1212 1120 DILOFLO¹(lb/MSF) 5.98 (0.457%) 7.98 (0.688%) 7.18 (0.592%) 8.99 (0.803%) Regularstarch (lb/MSF) 5.0 (0.38%) 0 4.6 (0.38%) 0 Pregelatinized corn 2.0(0.15%)  10 (0.86%) 2.5 (0.21%) 9.0 (0.80%) starch (lb/MSF) Sodiumtrimetaphosphate 0.7 (0.05%) 2.0 (0.17%) 0.6 (0.05%) 1.6 (0.14%)(lb/MSF) Total water/stucco 0.79 0.77 0.86 0.84 ratio (w/s) Trialformulation test results Dry board weight 1619 1456 1553 1443 (lb/MSF)Nail pull resistance (lb) 81.5^(†) 82.4 80.7 80.4 Flexural strength,41.7 43.7 44.8 46.9 average (MD) (lb) Flexural strength, 134.1 135.5 146137.2 average (XMD) (lb) Humidified bond² load, 19.2 17.7 20.9 19.1average (lb) Humidified bond^(2,3) 1.6 0.1 0.5 0 failure (%) ^(†)ASTMstandard: 77 lb MD: machine direction XMD: across machine direction¹DILOFLO is a 45% Naphthalensulfonate solution in water ²90° F./90%Relative Humidity ³It is well understood that under these testconditions, percentage failure rates <50% are acceptable.

As illustrated in Table 5, Trial Boards 1 and 2 were made from a slurryhaving substantially increased amounts of starch, DILOFLO dispersant,and sodium trimetaphosphate, while slightly decreasing the w/s ratio, incomparison with the control boards. Nevertheless, strength as measuredby nail pull resistance and flexural testing was maintained or improved,and board weight was significantly reduced. Therefore, in this exampleof an embodiment of the invention, the new formulation (such as, forexample, Trial Boards 1 and 2) can provide increased trimetaphosphateand starch formulated in a usable, flowable slurry, while maintainingadequate strength.

Example 6 ½ Inch Ultra-Light Weight Gypsum Wallboard Plant ProductionTrials

Further trials were performed (Trial Boards 3 and 4) using Formulation B(Example 1) as in Example 2, except that the pregelatinized corn starchwas prepared with water at 10% concentration (wet starch preparation)and a blend of HYONIC PFM soaps (available from GEO Specialty Chemicals,Lafayette, Ind.) was used. For example, Trial Board 3 was prepared witha blend of HYONIC PFM 10/HYONIC PFM 33 ranging from 65-70% byweight/35-30% by weight. For example, Trial Board 4 was prepared with a70/30 wt./wt. blend of HYONIC PFM 10/HYONIC PFM 33. The trial resultsare shown in Table 6 below.

TABLE 6 Trial Board 3 Trial Board 4 (Formulation B plus (Formulation Bplus HYONIC soap blend HYONIC soap blend Lab test result 65/35) (n = 12)70/30) (n = 34)* Board weight 1106 1013 (lb/MSF) Nail pull 85.5 80.3resistance^(a) (lb) Core hardness^(b) >15 12.4 (lb) Flexural strength,55.6 60.3¹ average^(c) (MD) (lb) Flexural strength, 140.1 142.3¹average^(d) (XMD) (lb) *Except as marked. ¹n = 4 MD: machine directionXMD: across machine direction ^(a)ASTM standard: 77 lb ^(b)ASTMstandard: 11 lb ^(c)ASTM standard: 36 lb ^(d)ASTM standard: 107 lb

As illustrated in Table 6, strength characteristics as measured by nailpull and core hardness were above the ASTM standard. Flexural strengthwas also measured to be above the ASTM standard. Again, in this exampleof an embodiment of the invention, the new formulation (such as, forexample, Trial Boards 3 and 4) can provide increased trimetaphosphateand starch formulated in a usable, flowable slurry, while maintainingadequate strength.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention. Itshould be understood that the illustrated embodiments are exemplaryonly, and should not be taken as limiting the scope of the invention.

1. A light weight gypsum wallboard comprising: a set gypsum compositionformed between two substantially parallel cover sheets, the set gypsumcomposition made using a gypsum-containing slurry including water,stucco, pregelatinized starch, and a naphthalenesulfonate dispersant,wherein the pregelatinized starch is present in an amount from about0.5% by weight to about 10% by weight based on the weight of stucco. 2.The light weight gypsum wallboard of claim 1, wherein thenaphthalenesulfonate dispersant is present in an amount from about 0.1%to about 3.0% by weight based on the weight of stucco.
 3. The lightweight gypsum wallboard of claim 1, wherein the naphthalenesulfonatedispersant is present in an amount from about 0.25% to about 2.0% byweight based on the weight of stucco.
 4. The light weight gypsumwallboard of claim 1, wherein the naphthalenesulfonate dispersant ispresent in an amount from about 0.3% to about 0.7% by weight based onthe weight of stucco.
 5. The light weight gypsum wallboard of claim 1,wherein the naphthalenesulfonate dispersant is in the form of an aqueoussolution containing about 40% to about 45% by weightnaphthalenesulfonate and the aqueous solution is present in an amountfrom about 0.5% to about 2.5% by weight based on the weight of stucco.6. The light weight gypsum wallboard of claim 1, wherein thenaphthalenesulfonate dispersant is in the form of an aqueous solutioncontaining about 40% to about 45% by weight naphthalenesulfonate and theaqueous solution is present in an amount from about 0.5% to about 2.0%by weight based on the weight of stucco.
 7. The light weight gypsumwallboard of claim 1, wherein the naphthalenesulfonate dispersant is inthe form of an aqueous solution containing about 40% to about 45% byweight naphthalenesulfonate and the aqueous solution is present in anamount from about 0.7% to about 1.5% by weight based on the weight ofstucco.
 8. The light weight gypsum wallboard of claim 1, furthercomprising sodium trimetaphosphate present in an amount from about 0.12%to about 0.4% based on the weight of stucco.
 9. The light weight gypsumwallboard of claim 1, wherein the wallboard has a dry weight of about1000 lb/MSF to about 1400 lb/MSF.
 10. The light weight gypsum wallboardof claim 8, wherein the wallboard has a dry weight of about 1000 lb/MSFto about 1400 lb/MSF.
 11. The light weight gypsum wallboard of claim 8,wherein the wallboard has a dry weight of about 500 lb/MSF to about 1000lb/MSF.
 12. A method of making light weight gypsum wallboard, comprisingthe steps of: (a) mixing a gypsum-containing slurry including water,stucco, pregelatinized starch, and a naphthalenesulfonate dispersant,wherein the pregelatinized starch is present in an amount from about0.5% by weight to about 10% by weight based on the weight of stucco, andwherein the naphthalenesulfonate dispersant is present in an amount fromabout 0.1% to about 3.0% by weight based on the weight of stucco; (b)depositing the gypsum-containing slurry on a first cover sheet; (c)placing a second cover sheet over the deposited slurry to form a gypsumwallboard; (d) cutting the gypsum wallboard after the gypsum-containingslurry has hardened sufficiently for cutting; and (e) drying the gypsumwallboard.
 13. The method of claim 12, in which the first cover sheetand the second cover sheet are made of paper.
 14. The method of claim12, wherein the slurry further comprises sodium trimetaphosphate presentin an amount from about 0.12% to about 0.4% by weight based on theweight of stucco.